1. Field of the Invention
This invention relates to a method and apparatus for providing additional supply air in a controlled manner into a permanently inertized room in which a predefined inertization level must be set and maintained within a specific control range.
2. Background Information
An established practice for reducing the risk of a fire in enclosed spaces such as EDP areas, electric switching and power-distribution compartments, sealed-off systems, or storage areas for particularly valuable commodities, has been to permanently inertize them. The preventive effect resulting from such permanent inertization is based on the principle of oxygen displacement. Normal ambient air is known to consist of about 21% by volume of oxygen, about 78% by volume of nitrogen and about 1% by volume of other gases. In order to effectively reduce the risk of a fire developing in a protected area, the so-called “inert-gas technique” is applied to correspondingly reduce the oxygen concentration by injecting into the room concerned an inert gas such as nitrogen. In terms of a fire-extinguishing effect, that level for most combustible solids is known to be reached when the proportional oxygen content has dropped to below 15% by volume. Depending on the specific combustible materials located in the protected area, it may be necessary to reduce the oxygen content even further, for instance to 12% by volume.
In other words, permanent inertization of the protected area down to a so-called “inertization base level”, where the proportional oxygen content in the air of the protected area has been reduced to 15% by volume, effectively minimizes the risk of a fire developing in that protected area.
The definition of an “inertization base level” as used herein generally refers to an atmosphere in the protected area which, compared to the oxygen concentration in normal ambient air, is oxygen-depleted, although for medical reasons the oxygen reduction would not be such as to pose a hazard to humans or animals, allowing these to enter the protected area at least briefly and perhaps after taking certain precautions depending on the circumstances. As indicated above, the primary purpose of setting the inertization base level at an oxygen concentration for instance of 13% to 15% by volume is to reduce the risk of a fire developing in the protected area.
In contrast to the inertization base level, the so-called “fully inertized level” corresponds to a proportional oxygen content in the atmosphere of the protected area that has been reduced to a point where effective extinction of a fire begins to take place. Thus, compared to the oxygen content at the inertization base level, the term “fully inertized level” reflects an even lower oxygen concentration at which the combustibility of most materials has already been reduced to a point where an ignition is no longer possible. As a rule, depending on the fire load in the protected area concerned, the fully inertized level is reached at an oxygen concentration of around 11% to 12% by volume. It follows that permanent inertization of the protected area at the fully inertized level not only reduces the risk of a fire developing in the protected area but actually serves to extinguish a fire.
For permanently inertized rooms it is desirable, on the one hand, to build them in relatively air-tight fashion, allowing the inertization level set or to be set to be maintainable with a minimum of inert-gas replenishment. On the other hand, a certain minimum air exchange is generally indispensable even for permanently inertized rooms so as to permit a regeneration of the room atmosphere. For rooms occasionally entered by persons, or occupied by persons for extended periods, that minimum air exchange is needed to allow adequate venting for instance of the carbon dioxide exhaled or the moisture given off by these persons. Considering this example, it is evident that the minimum air exchange required for that room must necessarily be a function of the number of persons and the duration of their activity in the room, with especially the length of time being a variable factor.
To be sure, a minimum air exchange must be provided even for rooms that are essentially never or rarely entered by persons, for instance storage areas, archives or cable pits and ducts. In this case, the minimum air exchange is needed for exhausting potentially harmful components of the room atmosphere caused for instance by fumes emanating from equipment housed in the room at issue.
If the enclosure of the room concerned is sealed in nearly hermetic fashion as is usually the case especially in permanently inertized rooms, an uncontrolled air exchange can no longer take place. Enclosed spaces of that nature therefore make it necessary for a technical or mechanical ventilation system to provide that minimum air exchange. The term “technical ventilation” collectively refers to a venting system for drawing out hazardous substances or biological agents present in a room. In the case of rooms in which persons perform activities, the dimensioning of a technical ventilation system, especially the blower output, air exchange rate and air flow velocity, depends on the time-weighted average concentration of a substance in the room atmosphere at which any acute or chronic damage to a person's health is not to be expected. Venting the room permits an air exchange between the outside and the interior atmosphere. In general terms, the required minimum air exchange serves to remove toxic, hazardous substances, gases and aerosols to the outside and to inject needed substances, especially oxygen, into rooms in which people are present. The following description will refer to these toxic substances that are to be removed from the enclosed-space atmosphere through the minimum air exchange simply as “hazardous substances”.
Large rooms or rooms in which the atmosphere contains a large amount of hazardous substances are now typically equipped with a mechanical ventilation system that ventilates the room either continuously or at preset times. The ventilation systems usually employed are designed to feed fresh air into the object room and to draw out spent or polluted air. Depending on the intended application, these are systems providing a controlled air intake (so-called “added-air systems”), or a controlled return air exhaust (so-called “air exhaust systems”), or they are combination air intake and exhaust systems.
The drawback of using this type of ventilation system for permanently inertized rooms, however, is that due to the air exchange, it is necessary to continuously feed inert gas into the permanently inertized room at a relatively high rate in order to maintain the preset level of inertization. It follows that, when mechanical ventilation is employed, maintaining the atmosphere in a permanently inertized room at the inertization base level or fully inertized level requires the supply of relatively large amounts of inert gas per time unit, produced for instance by appropriate on-site inert-gas generators. These inert-gas generators must have a correspondingly high output capacity, which in turn increases the operating cost of permanent inertization. Moreover, to produce inert gas, these generators use up a relatively large amount of energy. Therefore, from the economic point of view, applying inert-gas technology whereby a room is permanently inertized at the inertization base level or the fully inertized level for minimizing the risk of fire, entails relatively high operating costs whenever the permanently inertized room requires that minimum air exchange.